A traditional technique for recovering precious metal(s) from precious metal-containing ore is by leaching the material with a cyanide lixiviant. As used herein, a “precious metal” refers to gold, silver, and the platinum group metals (e.g., platinum, palladium, ruthenium, rhodium, osmium, and iridium). Many countries are placing severe limitations on the use of cyanide due to the deleterious effects of cyanide on the environment. Incidents of fish and other wildlife having been killed by the leakage of cyanide into waterways have been reported. The limitations being placed on cyanide use have increased substantially the cost of extracting precious metal(s) from ore, thereby decreasing precious metal reserves in many countries. Cyanide is also unable to recover precious metals such as gold from refractory ores without a pretreatment step. “Refractory ores” refer to those ores that do not respond well to conventional cyanide leaching. Examples of refractory ores include sulfidic ores (where at least some of the precious metals are locked up in the sulfide matrix), carbonaceous ores (where the precious metal complex dissolved in the lixiviant adsorbs onto carbonaceous matter in the ores), and sulfidic and carbonaceous ores.
Thiosulfate has been actively considered as a replacement for cyanide. Thiosulfate is relatively inexpensive and is far less harmful to the environment than cyanide. Thiosulfate has also been shown to be effective in recovering precious metals from pretreated refractory preg-robbing carbonaceous ores and sulfidic ores. As used herein, “preg-robbing” is any material that interacts with (e.g., adsorbs or binds) precious metals after dissolution by a lixiviant, thereby interfering with precious metal extraction, and “carbonaceous material” is any material that includes one or more carbon-containing compounds, such as humic acid, graphite, bitumins and asphaltic compounds.
Where gold is the precious metal, thiosulfate leaching techniques have typically relied on the use of copper ions to catalyze and accelerate the oxidation of gold, ammonia to facilitate the formation and stabilization of cupric ammine ions and/or a pH at pH 9 or above to maintain a region of stability where both the cupric ammine and gold thiosulfate complexes are stable.
It is well known in the art that the catalytic effect of copper and ammonia in conventional thiosulfate leaching of gold is described by the following sequence of reactions.
Formation of the cupric ammine complex:Cu2+4NH3→Cu(NH3)42+  (1)Oxidation of gold by cupric ammine, gold complexation as the gold-thiosulfate anion, and reduction of the cupric ammine to cuprous thiosulfate:Au+Cu(NH3)42++5S2O32−→Au(S2O3)23−+Cu(S2O3)35−+4NH3   (2)Oxidation of the cuprous thiosulfate back to cupric ammine with oxygen:Cu(S2O3)35−+4NH3+¼O2+½H2O→Cu(NH3)42++3S2O32−+OH−  (3)
Summing equations (2) and (3) yields the overall thiosulfate leach reaction for gold:Au+2S2O32−+¼O2+H2O→Au(S2O3)23−+OH−  (4)
It can be seen from the above equations that copper and ammonia act as catalysts in that they are neither produced nor consumed in the overall leach reaction.
Cupper and ammonia can be a source of problems. Added copper tends to precipitate as cupric sulfide, which is speculated to form a passive layer on gold, thereby inhibiting gold leaching as well as increasing copper and thiosulfate consumption:Cu2+S2O32−+2OH−→CuS+SO42−+H2O   (5)Rapid oxidation of thiosulfate by cupric ammine also occurs, leading to excessive degradation and loss of thiosulfate:2Cu(NH3)42++8S2O32−→2Cu(S2O3)35+S4O62−+8NH3   (6)Loss of ammonia by volatilization occurs readily, particularly in unsealed gas-sparged reactors operating at pH greater than 9.2, leading to excessive ammonia consumption:NH4+OH−→NH3(aq)+H2O→NH3(g)+H2O   (7)Like cyanide, copper and ammonia are highly toxic to many aquatic lifeforms and are environmentally controlled substances.
Other problems encountered with thiosulfate leaching include difficulty in recovering gold out of solution as a result of the formation of polythionates, such as tetrathionate and trithionate, which adsorb competitively with gold onto adsorbents, such as resins. The formation of polythionates further increases thiosulfate consumption per unit mass of processed ore.